Assistance with hexcopter abnormal rotor vibration

This is puzzelling me.

I watched this new drone take off today after nearing the finish of the initial build (its new). I've had problem after problem with this craft and im about to kick it and set it on fire. The latest problem arises after it took off, and an extreme vibration overcame the #3 motor (it was visible and can easily be heard, almost felt). Fun fact - the vibration actually caused tension on the ESC and caused one of the motor connectors to come undone and promptly launched the self locking propeller about 30ft in the air.

When idling, all six motors function fine. When I tune the throttle up around 60%, this vibration occurs and makes the aircraft uncontrollable (at least until the prop spun off and I was able to rotate it into a landing).

Without the prop, the motor does not cause any abnormal vibrations even at high RPM.

I have replaced the prop.

I have replaced the motor.each result in the same effect.

I am in the process of narrowing the issue down to a damaged arm (thanks Gerard) or a possibly damaged ESC.

If anyone can explain how an ESC can cause vibrations, please let me know. What ive gathered from research is that if the ESC isnt able to output enough current to one set of the poles, it could cause interference with the shaft... is this practical? I will replace the arm and swap the ESC and edit the results into this post.

Ive uploaded this video to try and gather some opinions on what the issue might be. Any insight would be greatly appreciated, and if you have seen something like this and can offer a solution I will buy you a beer.

Thanks for your time guys,

Chris

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  • My guess:  a fracture or weakening of the arm. When the prop spins fast, the lift force is applied sideways and then perpendicular to the weakness. At some specific RPM the lift force is sufficient to deform the arm due to the weaking, but as the lift force disappears when perpendicular, the arm twists back.

    Or you may not have replaced the prop hub every time, causing this vibration.

    • I may owe you a beer.

      I swapped the ESC's and still have the issue. I took a more kenetic approach and felt the play in the arm and noticed it was more giving than the other arms. Upon further inspection, I noticed what appears to be two tears in the arm, albeit they are very small. I am going to replace the arm (as well as the #6 arm) with thicker arms I recent got in.

      I will let you guys know how it works out,

      good call gerard.

      2014-11-21 20.02.09.jpg

      https://storage.ning.com/topology/rest/1.0/file/get/3702533385?profile=original
    • Thanks for your input Gerard.

      Im looking into this as a possible ESC issue. I am going to swap the ESC with one from a working arm, as well as replace the arm and will update the discussion with the results.

      Appreciate the help.

      Chris

  • Have you checked the prop shaft for straightness?  Are your props the same size hole as the shaft?  If the prop is well balanced and you have tried moving the prop to a different motor and the vibration didn't change I would bet the shaft or really bad bearings.  Occasionally you can get a bearing that spins in the mount instead of shaft spinning the bearing.  That can cause a vibration too.

    • Ive replaced both the prop (with a brand new one fresh out of the package) as well as the motor. I went through all the screws on the frame and tightened everything down and still having the issue... Im starting to narrow this down to a potential problem with the ESC, but I dont fully comprehend how the ESC can cause a vibration...

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